Rickets is the childhood bone disease in which soft, poorly mineralised bones fail to develop properly, caused by deficiency of vitamin D, often worsened by low calcium intake. Without enough vitamin D the intestine cannot absorb calcium, so the growing skeleton stays soft: legs bow under body weight, ribs bead, the skull may flatten, and growth falters. It was once endemic in industrial cities where smoky air blocked sunlight, and it still appears where children get little sun, where exclusive breastfeeding continues without vitamin D drops, or where diets are severely restricted. Treatment with vitamin D and calcium corrects it, and prevention is simple: sensible sunlight, fortified foods, and supplements for at-risk infants. Also called rachitis; the adult form is osteomalacia.